Indirectly heated cathode for an electron discharge tube with an insulated heating element

ABSTRACT

An indirectly heated cathode for electron discharge tube having a refractory metal support, e.g. molybdenum or tungsten, the inner surface of which is coated with a thin layer of a metal of the platinum group to minimize oxidation of the refractory metal. The cathode is insulated from a heating element by a layer of alumina which surrounds the heating element.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority Marinus Antonius Maria Bakker; JohannesChristiaan Duran, Emmasingel, Eindhoven, Netherlands Jan. 29, 1 968 Jan.5, 1971 U. S. Philips Corporation New York, N.Y.

a corporation of Delaware, by mesne assignments Feb. 17, 1967Netherlands lNiiiiii'fLy atria) kiiitbfi Eat AN ELECTRON DISCHARGE TUBEWITH AN INSULATED HEATING ELEMENT 2 Claims, 1 Drawing Fig.

U.S. Cl 313/337, 313/217, 313/356, 313/355 Int. Cl H0lj 1/20,

l-lOlj 19/14 [50] Field ofSearch 313/311, 337, 340, 356, 217, 218, 355

[56] References Cited UNITED STATES PATENTS 1,719,988 7/1929 Myers313/340 2,733,378 1/1956 Aisenstein et a1. 313/337X 3,401,297 9/1968Feinleib 313/340 Primary Examiner-John W. Huckert AssistantExaminer-Andrew J. James Attorney-F rank R. Trifari ABSTRACT: Anindirectly heated cathode for electron discharge tube having arefractory metal support, e.g. molybdenum or tungsten, the inner surfaceofwhich is coated with a thin layer of a metal of the platinum group tominimize oxidation of the refractory metal. The cathode is insulatedfrom a heating element by a layer of alumina which surrounds the heatingelement.

PATENTEUJAN 5197! V 3553521 INVENTOR. MARINUS A.M.BAKKER JOHANNES CH.BURAN INDIRECTLY HEATED CATHODE FOR AN ELECTRON DISCHARGE TUBE WITH ANINSULATED HEATING ELEMENT The invention relates to an indirectly heatedcathode provided with a heating wire which is insulated by alumina froma support for the emissive material, while at least the surface of thissupport facing the heating wire consists of molybdenum.

lt is known that the tungsten or molybdenum of a heating wire for anindirectly heated cathode may be attacked by oxygen, in which event theW or M00 formed evaporates and reacts with the insulating materialconsisting of alumina. The metal is then released and the insulationresistance of the insulating material gradually decreases. According tothe known method, this is prevented by coating the tungsten ormolybdenum heating wire with a suitable metal of the platinum group andby fusing this metal to the heating wire by a electrical currenttransient and strong heat treatment. It is emphatically stated that onlymolten layers of these metals are effective,whereas nonmolten layers ofthesemet'als applied, for example, by electroplating may exhibit thesaid disadvantage to an even greater extent. In order to prevent theresistance of the heating wire from becoming unduly low, the layers mustbe extremely thin (at the most 1 11.). Platinum itself cannot be used,since at the high temperature of the heating wire it is dissolved in themolybdenum during the melting process, while rhenium has an unduly highmelting temperature so that it cannot be fused to the heating wire.

However, it also has been found that if the heating wire is coated witha molten layer of one of the suitable metals of the Pt group such asrhodium, iridium or ruthenium, the insulation resistance still decreasesin cathodes in which at least the surface of the support for theemissive material facing the heating wire consists of molybdenum,especially if the support has a positive potential with respect to theheating wire.

Many investigations have shown that this decrease in resistance is dueto oxidizing gases released in the tube, such as water vapor, which havean oxidizing effect on the molybdenum, in which event the molybdenumoxide may be reduced in the A1 0,, and the metal may be released, or anoxide of lower valency which is dissolved in the A1 0 In this case, theknown method described above cannot be used, because, owing to thegreater mass of the support, the process of melting the suitable metalsofthe Pt group cannot be carried out so rapidly that a reaction with themolybdenum can be avoided.

However, it has been found that in this case a favourable effect mayalso be obtained by the application of a nonmolten layer of one of thesaid metals of the Pt group if the thickness of such a layer is 3microns or more. A Pt layer having a thickness of 3 microns is found tohave a sufficient density. It has been found that due to the lowertemperature of the sup port, only such a small quantity of Pt isdissolved in the Mo of the support and conversely that this doesnot giverise to dif' ficulties during the life of the cathode. ln this case, thecomparatively thick metal layer does not give rise to difficultieseither with respect to the resistance of the support so that suchcomparatively thick layers can be used without any objection.

The invention is preferably carried out in conjunction with the knownmethod described.

The invention will be described more fully with reference to theaccompanying drawing, which is. a sectional view of a cathode accordingto the invention.

In the drawing, reference numeral 1 denotes the support which in thiscase entirely consists of molybdenum. Theheater body 2 consists of aheating wire 3 to which a layer 6 of rhodium, ruthenium or iridiumhaving a thickness of less than 1 u is fused by the known method. Theheating'wire 3 is coated with alumina.

The support 1 is provided with an emissive layer 4 which preferablyconsists of a mixture of nickel powder and alkaline earth metal oxides.The surface of the support 1 facing the heating wire 3 is coated with aplatinum layer having a thickness of 5 p.. Instead pf platinum, also oneof the other metals of the platinum group may be used for the layer 5The layer 5 15 preferably applied by absorption of a platinum suspensionor by electroplating.

A cathode according to the invention is particularly suitable for use incircuit arrangements in which a potential difference of more than 400 v.may occur betweenthe heating wire and the cathode support, the support 1being positive with respect to the heating wire 3.

Although only one embodiment of a cathode according to the invention hasbeen described, the invention also applies to differently shapedcathodes, for example, dispenser cathodes. Furthermore, the emissivematerial may also be applied to a tubular support. Instead of A1 0 MgOmay be used.

We claim:

1. An indirectly heated cathode for an electron discharge tube providedwith a heating wire which is insulated by means of a metal oxide from asupport foran emissive layer, at least the surface of this supportfacing the heating wire consisting of molybdenum, said surface'havingthereon a layer of at least one of the metals of the platinum grouphaving a thickness of at least Bu.

2. A cathode as claimed in claim 1, wherein the heating wire has a layerof a metal of the platinum group.

1. An indirectly heated cathode for an electron discharge tube providedwith a heating wire which is insulated by means of a metal oxide from asupport for an emissive layer, at least the surface of this supportfacing the heating wire consisting of molybdenum, said surface havingthereon a layer of at least one of the metals of the platinum grouphaving a thickness of at least 3 Mu .
 2. A cathode as claimed in claim1, wherein the heating wire has a layer of a metal of the platinumgroup.